All electronic devices emit more or less strong electromagnetic radiation during normal operation, with a spectral bandwidths ranging from very narrow bands to bands that cover wide zones of the electromagnetic spectrum. One consequence of these emissions can be evaluated in terms of “noise” or “interference” on other electronic equipment. The powers brought into play depend on the characteristics of the device in question.
The evolution of electronic technologies over recent years has certainly brought about an increase in radiated powers, as can be witnessed, for example, by the wide-scale diffusion of mobile phones and the tendency towards ever-increasing clock frequencies in digital components (microprocessors for example) and systems that use them (such as personal computers, various types of peripheral, etc.).
For these reasons, directives have been issued, e.g., at European Union level and by U.S. federal administrations, aimed at guaranteeing that electronic equipment has a certain level of immunity regarding radiation emitted by other devices and that each single device cannot emit beyond a certain power level.
With regard to the reduction of emissions due to high clock frequencies in digital systems, various methods exist that start from the presupposition of reducing the phenomenon at the origin.
For example, it is possible to control the wave form of clock pulses and/or the slew-rate or widen the spectrum (spread spectrum) of the clock signal. This technique, which is found to be the most efficient from many points of view, essentially aims at redistributing the energy of the clock signal over wider bands with respect to those the un-modulated clock would have. Instead of having a fixed frequency, the clock signal has a frequency that varies in a precise manner over time. Passing into the frequency domain, since the clock continuously changes its frequency, instead of the energy of the harmonics being concentrated at well-defined frequencies, it is distributed over a certain range, the amplitude of which depends on how much the modulation shifts the signal frequency, i.e. the index of modulation.
Naturally, the modulating signal must have a much lower frequency than the clock signal because the latter signal is intended to synchronize a digital system. Only if the variations in clock frequency are sufficiently slow does the functioning of the entire system remain uncompromised. In addition, when a digital system is designed, it should be dimensioned for the highest frequency that the clock reaches when it is modulated. As opposed to what happens with pulse shaping, in spread spectrum techniques the clock signal remains a square waveform and its wave shape thus remains unaltered.
The spread spectrum technique described allows realization of devices currently known as SSCGs (Spread Spectrum Clock Generator) or “dithered PLLs”, intended for use as clock signal generators in digital ASICs for example. It allows the electromagnetic emissions (EMI) of a quartz oscillator, for example, to be reduced by 10–20 dB at determined frequencies by “spreading” the frequencies over a broader frequency range. This technique has been known for some time, as is attested, for example, by U.S Pat. Nos. 4,546,331, 5,488,627, 5,610,955, 5,631,920, 5,736,893, 5,943,382 and 6,167,103.
In particular, this last document describes the solution of modulating the PLL feedback divider via a fixed, memory-mapped synthesizer. A triangular “cusp-like” modulating signal is used in an attempt to improve the borders of the clock's spread spectrum.